This invention relates to a twin-wire former for a papermaking machine which produces a fiber mat by dewatering a (fiber suspension) material by sandwiching the material between two wire screens (hereinafter "wire screen" is represented as "wire"). This invention particularly relates to improvements in a twin-wire former of the type in which a material is ejected out into a bottom wire which travels substantially horizontally; water is removed out of the material downwardly by means of the foils or the like arranged beneath the wire; and thereafter a top wire which is arranged to sandwich the material by approaching the bottom wire from above and travel downwardly along with the bottom wire on the circumference of a roll provided within the loop of the bottom wire and water is removed upwardly above the top wire.
In the conventional twin-wire former, where the material is sandwiched between two wires, the circumference angle of contact between the wires and the roll, (hereinafter "the wire contacting angle") is fixed. This conventional twin-wire former has a drawback in that formation is deteriorated when the condition of the material to be sandwiched between the wires, for example, thickness or consistency of the material deviates out of a certain range. In addition, in production of thick paper, which needs a large quantity of material to be sandwiched, part of the material is often disadvantageously rejected (toward the upstream side) out of the portion where the two wires come into contact and, travelling in the same direction, press the wet material (hereinunder the portion is represented as "wedge portion"), particularly when the speed of the paper machine is low.
On the other hand, with paper produced by what is called a Fourdrinier machine which forms a fiber mat by removing the water only downwardly with a single wire which travels horizontally, the paper surface facing the wire has fewer fines and less clay, because the fines and clay in the vicinity thereof are washed out. When twin wires are formed by providing another wire (top unit) over the Fourdrinier so as to remove the water upwardly also, it is possible to make the distribution of the fines and the clay in the thickness of the paper more uniform on both sides.
However, the conventional twin-wire former of this type cannot produce paper with good formation over a wide range of basis weight and running speed.